1. Field of the Invention
The present invention relates to a differential operational amplifier.
2. Description of the Related Art
A differential operational amplifier is generally used for an operational amplifier with increased noise resistance (see, e.g., Japanese Patent Application Laid-Open Publication No. 1995-86850). FIG. 9 shows a typical configuration of a differential operational amplifier. A differential operational amplifier 100 is constituted by including P-MOSFETs 101 to 108, N-MOSFETs 109 to 112, and a common-mode feedback circuit (CMFB circuit) 115.
A bias voltage VB3 is applied to the gates of the P-MOSFETs 101 to 103 and a bias voltage VB2 is applied to the gates of the P-MOSFETs 104 to 106 to constitute a current source in the differential operational amplifier 100. A bias voltage VB1 is applied to the gates of the N-MOSFETs 109, 110 serially connected to the P-MOSFETs 104, 106.
An electric potential between the P-MOSFET 104 and the N-MOSFET 109 is an output voltage VOUTP, which is one differential output, and an electric potential between the P-MOSFET 106 and the N-MOSFET 110 is an output voltage VOUTN, which is the other differential output.
The P-MOSFETs 107, 108 are serially connected to the P-MOSFETs 102, 105 and constitute a differential circuit. One differential input, i.e., an input voltage VINP is applied to the gate of the P-MOSFET 107, and the other differential input, i.e., an input voltage VINN is applied to the gate of the P-MOSFET 108. That is, the differential outputs VOUTP, VOUTN are changed depending on the differential inputs VINP, VINN.
The N-MOSFETs 111, 112 are serially connected to the P-MOSFETs 107, 108 and the N-MOSFETs 1019, 110, and a control voltage VBC output from the common-mode feedback circuit 115 is applied to the gates of the N-MOSFETs 111, 112.
The common-mode feedback circuit 115 controls and outputs the control voltage VBC such that a common-mode voltage (intermediate voltage) of the output voltages VOUTP, VOUTN of the operational amplifier 100 becomes a reference voltage COMVREF. That is, if the common-mode voltage VC is higher than the reference voltage COMVREF, the control voltage VBC is raised. Since the raising of the control voltage VBC increases the drain currents of the N-MOSFETs 111, 112, the output voltages VOUTP, VOUTN are lowered and the common-mode voltage VC is also lowered. Contrary, if the common-mode voltage VC is lower than the reference voltage COMVREF, the control voltage VBC is lowered. Since the lowering of the control voltage VBC decreases the drain currents of the N-MOSFETs 111, 112, the output voltages VOUTP, VOUTN are raised and the common-mode voltage VC is also raised.
In this way, the operational amplifier 100 is controlled such that the common-mode voltage VC of the output voltages VOUTP, VOUTN becomes the predetermined reference voltage COMVREF by controlling the voltage VBC applied to the gates of the N-MOSFETs 111, 112.
In the operational amplifier 100, the gate capacities of the N-MOSFETs 111, 112 are generally increased to reduce 1/f noise. FIG. 10 shows a typical configuration example of the N-MOSFET 111. To increase the gate capacity, as shown in FIG. 10, the N-MOSFET 111 can be constituted by connecting, for example, 10 N-MOSFETs 111a to 111j of a certain size in parallel. The N-MOSFET 112 has the configuration same as the N-MOSFET 111.
When the input voltages VINP, VINN input to the operational amplifier 100 have large amplitude, the fluctuations of the output voltages become large and the swing of the common-mode voltage VC becomes large. When the swing of the common-mode voltage VC is large, the fluctuation band of the control voltage VBC becomes large. When the fluctuation band of the control voltage VBC becomes large, the change amounts are increased in the drain currents of the N-MOSFETs 111, 112 and the current change amount is increased in the operational amplifier 100. Therefore, if the amplitude of the input voltages VINP, VINN is large, the distortion rate of the operational amplifier 100 is deteriorated.
Since the gate capacities of the N-MOSFETs 111, 112 are large, the current amount must be increased in the common-mode feedback circuit 115 to steadily operate the common-mode feedback circuit 115, and the current consumption of the operational amplifier 100 is increased.